Should I enable both my CPU and GPU in the CUDA preferences?
It depends if you want to use only your GPU for rendering or GPU + CPU. You will have to test what gives you a better rendering performance. Using a fast GPU with a slow CPU may result in longer render times than using the GPU alone, while a combination with fast CPU may improve the performance.

Currently you shouldn't use GPU + CPU rendering if you're rendering a scene with volumetrics in Cycles, because the CPU implementation of Cycles uses equi-angular sampling and the GPU implementation doesn't. This could result in visible tiling in the render with more noise being present in the GPU tiles.

The OpenCL options are only for AMD graphics cards in Blender. This may be a bit confusing since OpenCL isn't only for AMD products, but Cycles uses CUDA for Nvidia graphics cards, hence the separate configuration.